Altitude is one of four degrees of freedom available to rotary wing aircraft. In helicopters, altitude is regulated by controlling the collective pitch of the main rotor blades. Collective pitch refers to the angle at which an airfoil (i.e. a main rotor blade) passes through the air. This angle may be called the "angle of attack"; it is measured between the chord of the airfoil and the direction of the relative wind. By changing the collective pitch, an aviator adjusts the vertical lift and resultant altitude of the aircraft. Minimal collective pitch is used when hovering flight is desired at a given resultant altitude. Changes in collective pitch may be accompanied by automatic or manual adjustments in engine power and cyclic pitch so as to maintain normal engine rpm, to control rates of vertical climb and horizontal flight, to correct dissymmetry of lift in forward flight, and to avoid stalling.
While the precise details of individual systems may vary in their particular components and design characteristics, main rotor blade assemblies typically include pitch control rod assemblies. The control rod assemblies constitute means for translating control impulses from the aviator's movement of the collective and cyclic pitch control levers in the cock pit of the aircraft. Servomechanical power is typically used to provide the mechanical force required for rotating the control rods and adjusting the pitch or angle of attach of each main rotor blade.
The main rotor blades must rotate at high speeds, with tip speeds often in excess of 400 miles per hour, to generate sufficient lift to elevate the aircraft off the ground and to the desired altitude. The blade assemblies, including the pitch control rod assemblies, are subjected to large loads even in routine operations. The loads include high frequency harmonic motion from vibrational resonance, lead-lag oscillations and flapping, turbulence from airflow patterns and Coriolis forces, resultant lift and induced drag.
Because of the loads on the main rotor shaft, blades, and components of their constituent assemblies, maintenance is often a challenge and durability and reliability are abiding concerns. This applies to the pitch control rod assemblies as well as to other components of the main rotor system. The loads that the main rotor system are subject to can adversely affect the pitch control mechanism by allowing the control rods to elongate, thus forcing the rotor blades out of their track. Alternatively, the rotor blades may be free to rotate on the rod axis, unable to hold a pitch setting. A malfunction of either type could threaten the airworthiness of the craft so affected. In turn, mission effectiveness would be jeopardized and the aircraft and crew would be placed at risk in that field repair of such malfunctions would probably be difficult at best and practically impossible in most commonly encountered circumstances.
The present state of the art relating to pitch control rod assemblies encompasses two typical means of safeguarding against the type of mechanical malfunction alluded to above. To fully understand the nature of these safety means, it will be necessary to describe briefly the components and structure of a typical pitch control rod assembly. It is to such an assembly that the improvement provided by the instant invention, described in detail below, applies. Basically, a pitch control rod assembly comprises a barrel which is castellated at one end and two rod ends with elastomeric bearings which threadably engage the inside surface of the barrel ends. Once installed, the rod ends are oppositely displaced to one another. The distance between the center line of the rod ends may be adjusted by turning the barrel; when the desired distance is reached, jam nuts which threadably engage the outside surface of the rod ends are used to lock the adjusted positions of the rod ends. The castellated end of the barrel is machined to provide incremental adjustment notches for changing the center line distance between rod ends in relation to the barrel. A locking kay fits into a notch to hold a predetermined setting; the torque applied to tightening the jam nut holds the key in its designated notch. Adjustment of control rod length is made to provide for the most appropriate rotor blade track and balance. If torque on the jam nut loosens, say as a result of vibratory stress of maintenance error, the key disengages from the notch into which it has been placed for the desired adjustment and the rod is free to rotate and is unable to hold a pitch setting. The free twisting of the rod end when the jam nuts loosen also permits changes in control rod length which results in the rotor blade leaving its track.
This typical pitch control rod assembly of the prior art lacks a passive safety feature to guard against the hazards posed by loosening jam nuts or by jam nuts that have not been properly retorqued after control rod length setting or adjustment. Having passing locking redundancy in the assembly would ensure the structural integrity of the assembly. This invention provides an improvement to the combination pitch control rod assembly of the prior art by adding means to passively engage the locking key in its groove in the event that a locking jam nut loosens in flight or has not been properly torqued into place during routine service or repair.
In another of its aspects, the invention pertains to preload spring means for keeping the control rod barrel in tension even under compressive loads in order to prevent load reversal resulting from loose jam nuts. Such load or stress reversals can result in premature fatigue failures.